The invention relates to the use of epoxidized low viscosity polydiene polymers as toughening modifiers for epoxy resins. More specifically, blends of epoxy resins and epoxidized low viscosity-polydiene polymers are used for structural adhesives, coatings, especially as primers, electrical applications such as castings, coatings, encapsulants, potting compounds, solder masking compounds, and laminates and construction applications such as flooring, civil engineering, concrete repair and consolidation, secondary containment of tankage, grouts, sealants, and polymer concrete, and structural composites, and tooling, etc.
Cured epoxy resins are typically strong, rigid, hard materials. Further, because of their chemical constitution they adhere strongly to many substrate materials. These physical characteristics of cured epoxy resins make them useful in a broad range of applications. One disadvantage of cured epoxy resins is their brittle character. When subjected to impact, cyclic stresses, thermal stresses, or differences in adhesive-substrate expansivities, epoxy resins tend to fail at relatively low applied stresses in a brittle manner. The goal of much effort in this area has been to improve the toughness, or equivalently stated, the energy required to fracture, epoxy resins. Improvements in this regard lead to mechanically superior materials.
Therefore, it would be advantageous if an epoxy resin composition with increased toughness could be achieved. Importantly, the desired increase in toughness must occur with little or no sacrifice in the beneficial mechanical properties of epoxy resins such as strength, rigidity, hardness, and adhesion.
One route to this improvement is to incorporate a rubber into the epoxy matrix. Increases in toughness by incorporation of a rubber phase in an epoxy matrix are well known. Carboxy functional rubbers, as described in commonly assigned U.S. Pat. No. 3,823,107 entitled "Epoxy Resin Curing Agent Compositions, Their Preparation and Use," have been used as modifiers for epoxy resins. These carboxy functional modifiers suffer the disadvantage that they must be pre-reacted with the epoxy resin before cure so that useful improvements in properties are achieved. Anhydride or acid functional graft copolymers, as described in U.S. Pat. No. 5,115,019 entitled "Carboxy-Functional Hydrogenated Block Copolymer Dispersed in Epoxy Resin," and U.S. Statutory Invention Registration (T-4577), entitled "Epoxy Resin Composition," have been used as modifiers for epoxy resins. These rubbers also suffer the disadvantage that pre-reaction is required. Further, in some cases solvent blending and formation of emulsions of the polymeric modifier is required. The processes required to disperse these polymers possess the further disadvantage that the resulting dispersion of rubber in epoxy is sensitive to the process parameters such as temperature and shear rate during mixing, length of time of mixing, and type and amount of solvent so that inconsistent products are produced with varying properties.
A second disadvantage of epoxy resins is their propensity to absorb water leading to lowered glass transition temperatures and lessened mechanical properties. The objective of efforts in this area has been to reduce the amount of water absorbed by incorporating strongly hydrophobic materials into epoxy resins.
Low viscosity epoxidized polydiene polymers are known to be used in modification of epoxy resins. Such polymers are described in commonly assigned U.S. Pat. No. 5,229,464. These polymers are liquid epoxidized rubbers. Compatible blends of the polymers of the above-described patent and epoxy resins are described in allowed copending commonly assigned application Ser. No. 08/090,856, filed Jul. 12, 1993, U.S. Pat. No. 5,332,783, which is herein incorporated by reference. The blends described in the afore-mentioned patent application have the disadvantage that their compatibility with epoxy resins is limited. Their limited compatibility does not extend to a broad range of epoxy resins and curing agents. Compatibilizing curing agents are required. They have the further disadvantage that even when marginally compatible, these polymers do not yield final cured epoxy resins having improved toughness. Additionally, the compatibilizing curing agents lead to cured epoxy resins have significantly reduced rigidity which makes them applicable in only limited applications.
The low viscosity epoxidized polymers of the present invention yield novel cured epoxy resin compositions having a superior balance of properties. The beneficial properties of the cured epoxy resin composition such as strength, rigidity, and hardness are maintained at high and useful levels while the toughness is simultaneously increased. Therefore, the compositions of the present invention present a broad utility. Additionally, the epoxidized polymers of the present invention are simply blended with epoxy resins before cure with no pre-reaction or solvent required. Further, it is anticipated that the strongly hydrophobic character of the epoxidized polydiene polymers will reduce the amount of water absorbed by the rubber modified epoxy resin.